It is known from U.S. Pat. No. 4,813,783 to use a refractometer alongside a measuring interferometer in order to determine the refractive index of the atmosphere in the region through which the measuring beam of the interferometer passes. By this means it is possible to correct the measurements made by the interferometer for changes in refractive index which may occur during the measuring operation.
The system described in the U.S. patent referred to above, employs evacuable chambers through which each of the measuring and reference beams pass, and provides two interferometers which make measurements in parallel of the same object. Various distance measurements are made with the object at different positions and with different ones of the evacuable chambers either fully evacuated or open to the atmosphere. From the measurements the refractive index of the atmosphere is determined, and the various distance measurements are corrected for changes in the refractive index.
A known system for providing an absolute determination of the refractive index of the atmosphere is described in our European Patent Publication No. 508,583. In this publication there is described at least one auxiliary interferometer associated with a measuring interferometer, and through which a portion of the beam of the measuring interferometer is diverted. The diverted beam is split to form so-called reference and measurement arms which travel along different paths having a fixed, known path length difference before being re-combined.
An object of the present invention is to provide an alternative interferometric measuring apparatus in which a measuring interferometer is combined with a refractometer.